THE TITAN BALLOON
Jacques Blamont
Centre National d'Etudes Spatiales, 2, place Maurice Quentin, 75039 Paris Cedex 01, France ;
Jacques. blamontfa),cnes. fr
ABSTRACT
After a présentation of thé major scientifîc objectives of
Titan's exploration, thé montgolfière of thé TandEM
proposai to ESA is described, followed by thé
alternative option of a hydrogen balloon System.
1.
INTRODUCTION
After several close Cassini flybys, in January 2005,
thé Huygens Probe became thé first robot to
perform a descent through Titan's atmosphère onto
its surface during which it returned more than
3 hours of useful data.
Water ice constitutes 40 % of thé mass of Titan,
Saturn's largest satellite, and is thé dominant feature
of thé surface. An atmosphère of 95 % N2 and 5 %
CH4 (at thé ground) is maintained in thé gaseous state
by thé greenhouse effect of méthane at a surface
température of 94 K. Méthane to Titan is like water to
Earth in thé hydrological cycle. Discovering clouds,
lakes, fluvial and dendritic features attributed to CH4 in
its three phases, solid, liquid and gaseous, CassiniHuygens found that thé balance of géologie processes impacts, tectonics, fluvial, aeolian - is somewhat
similar to thé Earth's. Titan may well be thé best
analogue to an active terrestrial planet in thé sensé of
our home planet, albeit with différent working
materials : it is unique in thé Solar System with its
extensive, dense atmosphère, which bas turned out to be
a complex chemical reactor over an altitude range of
more than 1,000 km.
Even if thé solar flux is reduced to one hundredth of its
value at Earth, thé méthane is irreversibly
photodissociated into hydrogen, which escapes, and
molecular fragments which form not only higher order
hydrocarbons as ethane, acétylène and even benzène,
but also nitriles by combination with nitrogen, which
fall to thé surface. A sea of ethane should exist with a
depth of a few hundreds meters, but is not observed.
The life time of méthane is 30 million years, a very
short time by geological scales. The surprising présence
of méthane in thèse conditions and thé absence of thé
ethane sea hâve led to suppose thé existence of an
underground sea of liquid water containing an antifreeze
as dissolved ammonia, at a depth of tens of kilomètres.
To support or discard this hypothesis is one of thé main
objectives of Titan science, being it understood that thé
major problems to be solved are related to thé cycle of
méthane - as on earth and Mars, they are related to thé
cycle of water.
An important limitation of Cassini as concerns Titan
science has been thé insufficient spatial coverage of its
orbit. While thé measurements hâve highlighted thé
complexity of Titan's atmosphère and magnetic
environment, thé coverage has been insufficient to
actually understand them. The minimum altitude of
950 km as well as thé uneven horizontal coverage has
limited thé in situ atmosphère measurements,
opportunities for occultation hâve been very rare and
big gaps are remaining in thé magnetospheric
downstream région. Thus, in spite of thé incontestable
breakthroughs, many new questions hâve arisen from
thé discoveries of thé Cassini-Huygens mission.
For thé spécifie science goals, thé Cassini-Huygens
payload and orbital tour were either not optimized or
adéquate. Huygens was not developed as a lander but as
a descent module. Even its in situ measurements of thé
atmosphère are limited to just one vertical profile.
The limitations of Cassini-Huygens and thé questions
that will remain to be answered hâve led a consortium
of Titan and Enceladus experts, led by Athena
Coustenis, to propose a mission to thé Saturnian System
which includes an aerostatic station floating in thé
atmosphère of Titan [1]. This proposai, first called
TandEM and later TSMM, was presented to ESA in thé
frame of thé Cosmic Vision program, and rejected.
Its primary science goal was to improve our
global understanding of Titan's surface, interior and
atmosphère, détermine what kind of pré- and protobiotic chemistry may be occurring on Titan, and learn
about thé satellite's origin and évolution.
To achieve thèse objectives, thé System should
perform in situ measurements in thé atmosphère, at
very low altitude for a long period of time, in order to :
a) obtain a large body of data on thé atmosphère itself.
b) image at thé meter scale, access, sample and
analyze thé varions surface features (such as thé
hydrocarbon lakes, thé aérosol and organic
deposits, thé dunes, river Systems, mountain
___________________________________________________________________________________
Proc. ‘19th ESA Symposium on European Rocket and Balloon Programmes and Related Research,
Bad Reichenhall, Germany, 7–11 June 2009 (ESA SP-671, September 2009)
ranges and volcanoes), inciuding thé solid surface
and subsurface material.
c) caracterize thé chemical and isotopic nature of thé
atmosphère, of thé aérosols of thé largest number
possible of thé matter covering thé ground and of
thé liquids constituting thé lakes.
d) détermine whether Titan bas a sub-surface liquid
océan. An explanation is in order hère because of
thé new nature of thé method to be employed.
Cassini-Huygens discovered a strong ELF
émission at 36 Hz interpreted as thé second
eigen-mode of thé cavity foraied by thé
ionosphère at thé top and a conducting boundary
situated below thé surface. The phenomenon,
called Schuman's résonance, could be excited by
a plasma instability mechanism associated with
thé corotating saturnian magnetosphere. The
electric field of thé émission would hâve to be
mapped in order to verify thé hypothesis and
eventually characterize thé "océan" [2].
e) détermine thé value of thé magnetic fîeld, if it
exists. The magnetic fîeld is another way to detect
an underground océan.
2.
WHY A BALLOON ON TITAN ?
Withstanding thé compelling scientific reasons just
exposed (thé mapping of thé Schumann résonances and
thé détermination of thé magnetic fîeld can only be
obtained by instruments placed on a balloon), it happens
that Titan is thé best place for scientifîc ballooning in
thé solar System [3,4].
Therefore thé effect of differential molecular
mass between thé buoyant gas and thé ambient is
maximized.
2. The low value of soîar radiation (1Q~2 of radiation at
Earth) créâtes no diurnal change in thé external
energy source and opens thé possibility of long
duration flights.
3. Because of thé thickness of thé atmosphère, thé
inflation during descent is easy ; it can be started at
a vertical velocity of 5 ms"1 around 30 km of
altitude (20 mbar pressure) down to 3 km within a
number of hours (compared to a velocity of
30 ms"1 for thé Martian balloon).
It was recognized in 1978 at thé Service d'Aéronomie
du CNRS that montgolfières could fly for a long time
in thé Earth's atmosphère, and I proposed to extend
this concept to thé exploration of Titan (Tec. Note 675
CNES/HC, 7-2-78).
After thé détermination of thé atmosphère model by
Voyager I, more studies were made of Titan balloons ;
a fîrst proposai was put forward in 1983 - and
repeated on various occasions without success, for a
hélium fîlled vinyl fluoride open balloon of 6 meter
diameter. Total System mass was 41 kg.
Since 2000, JPL bas conducted studies, rediscovered
old concepts, introdueed thé idea of modem MMRTG
and developed material suitable for low température
aérostats.
3.
ÎSÛ •
THE TITAN MONTGOLFIERE
The problems which may be encountered by a balloon
as a vehicle in Titan's environment are :
» The resources it carries (lifting power, energy) are
not renewable.
• The gas providing buoyancy bas to be brought
inside tanks whose mass is of thé order of ten times thé
mass of gas (could go down to 5-6 in récent
developments).
ÊQ
?o
ao
9n
103
ito 12& ÎSD uo TSQ
ieo iro
Température (K)
Figure 1 : Température versus altitude is shown for
Titan's atmosphère. The solid Une represents thé
températures measured by thé HASÎ instrument on thé
Huygens probe [5], whereas thé symbols arefrom thé
Voyager radio occultation data [6]. The horizontal
Une shows thé base of thé Huygens-inferred méthane
cloua [7],
l. Its atmosphère is cold and dense with a ground
température of 94 K and a pressure of 5 kgm"3 at thé
ground level compared to 1 kgm"3 on Earth.
Thèse two constraints favour thé choice of hot air
balloons, or montgolfières which, using thé ambient air,
need no tanks to be filled.
This choice is supported by a second reason, thé
necessity to use RTG for electric power. Its large
thermal loss becomes a benefit since it could constitute
thé energy source for heating thé internai gas and
providing free lift.
The physics of thé montgolfière is dominated by beat
exchanges with thé ambient atmosphère, essentially due
to convection. Radiation exchanges are negligible.
hâve been used since 1979 by CNES in its baîloon
program, with a launch of 2 to 5 long duration balloons
nearly every year. For a fifty kg payîoad, theîr volume
lies in thé range of 50,000 ni3. A large body of
expérience bas been accumuiated.
Gondola
Figure 2 : Principle ofthe RTG heated montgolfière
Ai Titan, significantly less beat is required than on Earth
to provide thé same buoyancy with thé same size
balloon. The cryogénie environment at Titan results in
lower convective and radiative beat transfer
coefficients, reducing beat loss from thé balloon surface
and also greater buoyancy for a given température
différence between thé balloon internai température and
thé ambient température.
Studies bave been made at APL and JPL in thé scope of
a NASA Flagship mission to Titan inciuding a RTGheated montgolfière [8].
Q.5-meter V§nî
Altitude Control
11,5-meter
Monîgsîfisra
RPS (2000 watts)
As a support for thé TandEM mission Guillaume Mas
and Jean Marc Charbonnier at thé CST performed in
2007 an analysis of options relative to thé configuration
of a montgolfière plus a hélium filled auxiîiary balioon
supposed to provide extralift if thé convection with thé
ambient atmosphère would be large and therefore
reduce thé buoyancy. Their preferred option had a
1,000m3 montgolfière (radius 6.2 m) and a 50m3
ballonet (radius 4.6 m) with 125 kg for thé jettisonable
gas tanks. Thèse balloons sustained a 120 kg gondola
(80 kg for thé RTG, 25 kg of instruments),
corresponding to a total mass of 350 kg.
The JPL solution for overcoming convection losses was
différent : it used a double wall for thé montgolfière
envelope, qualified by a nuinber of flîghts carrying
aeronauts. This solution is lighter (276 kg for thé
montgolfière) and simpler, and therefore was adopted
by thé TandEM Team for its final proposai. The total
mass charged for thé aerial piatform at iaunch was
600kg. Cryogénie balloon material consisting of a
polyester film and fabric laminate, was developed by
JPL and a prototype bîimp (length 7 m) was built by
JeffHallandal[10].
The MMRTG is located inside thé balîoon just
above thé bottom opening, The science payîoad
itself is included m thé gondola, which is suspended
beneath thé balloon and provides unobstructed views
of Titan's surface and horizon for seientiflc
observations.
Figure 3 : Titan RPS montgolfière with altitude control
(after J. Jones and al) [9]
The beat generated by thé MMRTG during thé émise
phase is dissipated through a radiator with 2.5 m2
surface. The radiator éléments hâve been placed on
thé support structure for an optimal view factor to
space and to make use of an already existing
structure. The height was sized such that thé required
area (2.5 m2) could be accommodated on thé
circumference, taking into account continuity at thé
panel edges for thé routing ofthe fluid fines.
A montgolfière is an open balloon with an aperture at its
bottom, filled with ambient gas. A venting valve is
placed at thé top. The internai gas is heated and
therefore less dense that thé ambient ; thé température
differential provides buoyancy. The drawback ofthe bot
air balloon is that, because this differential is smail, its
dimensions bave to be very large. Such vehieles, using
as an energy source thé solar radiation during thé day,
and thé infrared émission ofthe ground during thé night,
The MMRTG needs integrating at thé final stages,
The montgolfière system bas therefore been split into
three major sub-assembîies, which are ail connected
at thé three mounting points at thé side of thé main
piatform :
1. front beat shield,
2. main piatform, and
3. back cover inciuding back-shield, parachutes and
balloon.
Insyîatîon
2'àxis Glffibaled
O.Ô-m&ter psarnetsr
Rotation Disk
Payioad
depending on thé choice of thé atmospheric profile.
The release of thé montgolfière occurs about 1.5 2 hours after entry. The diameter of thé main
parachute is 9 m, as required for a clean séparation
of thé 2.6 m diameter heat shield. The terminal
velocity of thé parachute is 6.5 m/s, which is
compatible with thé deployment and filling of thé
balloon.
- Balloon diameter in m x 10
- Baîloon mass in kg
Montgolfière deployment and fillûii
altitude : 30 km, 5 m/s descenî
Montgolfière filtad
1.5 m/s < faîl spssd <
150
Fîighî altitude
controled by a
<U piloted valve
attitude : 10 km
Gondola mass in kg
Figure 4 : Relation between balloon diameter/mass
and gondola mass
In this way thé three assemblies can be supportée!
individualiy allowing a iate intégration of thé
MMRTG, and subsequently relatively simple final
assembly steps. The MMRTG needs to be connected
to thé main platform for support during launch loads,
and to thé support cabling of thé balloon, such that it
can be pulled into thé balloon during deployinent.
Table 1
Elément
Mass in kg
POIS (payload to orbiter
interface System)
93
EDIS (entry descent and
inflation System)
202
Balloon
132
Gondola 144
Montgolfière total
276
Total launch mass
571
Launch margin
29
Allocated mass
600
Opérations
The montgolfière will be targeted at about 20°N,
where thé zonal wind has a predicted maximum with a
speed of a few ms"1 for thé time of arrivai in thé year
2030. The lifetime will be at least six month, which
corresponds to one circumnavigation around thé globe
with winds at 1 ms"1.
The inflation is a criticai phase in any planetary
balloon mission, but in Titan's thick atmosphère it
should présent no major difficulty ; thé modélisation
shows a time of about 12 hrs for reaching thé ceiling.
The release of thé main parachute is triggered by a
décélération event. For thé montgolfière thé altitude
of this release is at about 130 km ± 20 km
•a
Time (h)
Figure 5 : Titan deployment andflight simulation
(in thé configuration wifh a hydrogen balloneî)
At an altitude of about 40 km (measured by a
pressure gauge, and using an assumed altitude
pressure relation) thé balloon will be pulled out, and thé
MMRTG will be pulled inside thé balloon at thé same
time. After having achieved sufficient buoyancy,
thé float altitude of 10 km will be actively
maintained within a range of ± 2 km by a vent
valve placed at thé top, which will be controlled
by a pressure sensor for altitude measurement,
The data are retrieved through a relay satellite. The
buoyant montgolfière will slowly drift around Titan.
During this time thé orbiter is still performing Titan flybys during its séquence towards thé final
observations orbit. The distance to thé orbiter varies
between 5 x 106 km and a few 1,000 km during thé
nominal lîfetime of thé montgolfière. The distance to
thé orbiter is shown as a fonction of time. In this
figure, thé total évolution of distance is shown.
Periods where thé orbiter is above an élévation of 20°
are piotted with full lînes. ït can be seen that thé
orbiter cornes significantly closer during short
intervais, which provides much higher telemetry
capability.
The orbiterfs télécommunication System includes a
steerable 4 m diameter HGA with a multiple frequency
capability, which will allow using thé saine telemetry
and telecommand System for thé montgolfière and thé
lander included in thé Tandem mission. The
communications link will be in X-band at 8.45 GHz.
The montgolfière has a 50 cm2 degrees-of-freedom
steerable HGA with an antenna gain of 31 dB. A
pointing accuracy of 1° was assumed. The position to
thé orbiter will be measured by using a beacon signal
that will be emitted by thé orbiter. A coarse position
détermination will be performed by a phase based line
of sight measurement, and a fine pointing ;
measurement will be performed by a narrow angle
antenna scan.
Table 2
ITT situ
éléments
DhlariiV to orhitor
VA
A
Overall
dimensions
Front shield : 2.6 m 0
Balloon : 10.5 m 0
Gondola: 1.6 m 0
Interface mas s
571kg
Payload mass
21.5kg
Model
* Visible imaging System
(0.4-0.7 |um, ineluding
stéréo vision)
* Imaging spectrometer
(1 - 5.6 jim)
* Chemical analyzer
(10 - 600 Da mass spectrometer)
* Atmospheric structure
instrument/ meteorological package
* Electric environment package
* Magnetometer
• Radar sounder(> 150 MHz)
* Radio science using
montgolfière télécommunication
system
Power system
MMRTG(100W d )
Operational
lifetime
6 months (baseline)
+ 6 months (extended)
I ÎTIIC in vlu\ ;itu*r enlr>
Figure 6 : Distance between montgolfière and
orbiter. The évolution ofthe distance is plotted with a
dashed line; periods when thé orbiter is above 20°
élévation (typical useful limitfor
télécommunications) are drawn withfull line.
Vloiiliîolllcf l'rynsnnssion I^ulc
Communications
5.
I imc siriec \y idavsj
Figure 7 : Theoretical data transmission ratefrom thé
montgolfière to thé orbiter assuming a link margin of
3 dB, and minimum élévation of30°.
The theoretical capability of thé telemetry link to thé
orbiter ranges from a few 10 kbps to > 100 Mbps. At
higher levels, thé processor and transponder capabilities
would likely be saturated. To make thé most optimum
use of this large variation of link capability, a variable
transmission data rate will be implemented.
4.
THE PAYLOAD
Tab. 2 présents thé characteristics of thé balloon system,
ineluding a model payload and thé mass breakdown of thé
mission. The priority is given to thé GCMS for chemical
analysis and to thé caméra for ground pictures. However thé
détermination of thé magnetic field and of thé Schumann
résonances for thé détection of an underground océan is also
considered as essential.
One aerial vehicle (montgolfière)
floating at mid latitudes
(10 km altitude)
X - b a n d HGA 50 cm 0,
55 W T W T A
SURFACE OPERATION CAPABILITY
No surface opérations were contemplated in thé frame of
Tandem. However, since this mission has not been
accepted by ESA/NASA for a launch before 2020 in thé
Cosmic Vision program, it may be interesting to study
other concepts for thé future exploration of Titan.
The objective of surface measurements would stay
unchanged but with more emphasis on thé understanding
of thé organic chemistry of thé crust. In order to collect
sample, a guide rope similar to thé "snake" ofthe RussianFrench Mars balloon seems adéquate. Such a snake would
move slowly on thé ground for a while and thé balloon
would climb again in altitude, repeating this cycle with a
period of a number of hours. The snake would collect
samples and analyze them with its own detector, or hâve a
way to carry thé samples to thé gondola.
Tether îo Balloon
- Rope Tail
Time (h)
Figure 8 : Baselme guide-rope configuration
(thé snake ofthe Mars-94 French-Russian mission)
Figure 10 : Modélisation ofthe motion ofthe TandEM
montgolfière with an argon ballonet
The problem is thé création of such a periodic motion
whieh would proteet thé montgolfière from any contact
with thé ground. A possible method is thé use of thé
venting valve whîch could be commanded by a laser range
finder in order to maintain thé altitude around 100 meters,
as has been suggested at JPL.
would sustain a 12 kg gondola in a 30 m3 balloon. Both
thé argon and hydrogen baîloons would never be fully
inflated, thé System oscillating between 0 and 10 km of
altitude with a period of tens of hours. A modification
of thé buoyancy could be obtained by an on/ofif
circulation ofthe hydrogen around thé MMRTG.
Another method would be thé addition of a balloon filled
with argon. In Titan's atmosphère, argon goes from thé
vapour to thé lîquid phase at thé altitude of 3km,
providing a change in buoyancy equal to p^ Yb (Patm is
thé atmospheric density, Vb is thé volume of thé balloon
containing argon).
Such a mission would be thé culmination of scientific
ballooning since it was started by Gay-Lussac in 1804.
6.
REFERENCES
1.
Coustenis A. et al, TandEM : Titan and Enceladus
mission, Exp. Astron. (2009) 23:893-946.
Beghin C et al, A Schumann-lïke résonance on
Titan driven by Saturn's magnetosphere. Icarus,
(2007) 191,257-266,
Biamont J., Planetary balloons, Exp. Astron.
(2008)22:1-39.
Lorenz R.D., A review of balloon concepts for
Titan, JBIS (2008) 61,2-13.
Fulchignoni M. et al, Titan's physical
characteristics measnred by thé Huygens
atmospheric structure instruments, Nature (2005)
438,785-791.
Lîndel G,F., The atmosphère of Titan from
Voyager I radio occultation measurements, Icarus
(1983) 53, 348-363.
Tokano J. et al, Méthane drizzle onTitan, Nature,
(2006) 442,432-435.
Leary J.C. (study lead), Titan Explorer Flagship
Mission Study, NASA, (2007) 07-05735, NASA
Washington, D.C.
Jones J. and Wu J.J., Montgolfière aerobots for
Titan, Internat Planet Probe Workshop, Pasadena
(2007).
Hall J.C., Kerzhanovich V.Y., Lachenmeier T.,
Mahr P., Pauken M., Plett G.A., Smith L., Van
Luvender M.L., Yavroman A.H., Expérimental
results for Titan aerobot thermomechanical
subsystem development, J. Adv. Space Res. (2007),
DOI 10-1016/j.asr 2007.02.060.
2.
3.
s
_(
j_,__
4.
5.
6.
Figure 9 : Van 't Hoffdiagramfor argon m Titan 's
atmosphère (pressure in ordinates, inverse ofthe
température in abcissae)
For instance a mass of 10 kg argon would fill a ballonet
of 1.5 m3 (mass 0.3 kg) at thé altitude of 3 km. Its
vaporization would provide 7 kg of lift.
The main bailoon in a "smali mission" could be a
hydrogen filled balloon. On Titan, an open balloon
filled with hydrogen would hâve a lifetime of a few
weeks because ofthe low température. A mass of 10 to
20 kg of hydrogen (carried to Titan in a 100 kg tank)
7.
8.
9.
10.